The present invention relates generally to geophysical exploration and, more particularly, to methods for identifying and evaluating stratigraphic prospects in seismic data associated with gradational changes in elastic impedance.
In the continuing search for hydrocarbons contained in the earth's subterranean formations, explorationists and geophysicists have developed numerous techniques for imparting seismic wave energy into the earth's subterranean formations; recording the returning reflected and refracted seismic wave energy; and processing the recorded seismic wave energy to produce seismic signals or traces. Such seismic signals or traces contain reflection events representative of the structure of the earth's subsurface formations. The reflection events can also include a multiplicity of information, for example, frequency, amplitude, phase, etc., which can be related to stratigraphic features of the formation (e.g., lithology and pore fluid content). Recently, geophysicists' interest has turned to evaluating high intensity amplitude and variations in a seismic signal amplitude as a function of range or offset for selected reflection events. Exemplary of such focus are Bodine U.S. Pat. No. 4,779,237 and Bodine et al., U.S. Pat. No. 4,646,239. Such bright spot analysis techniques have been used primarily as a pore fluid indicator, and especially as an indicator of gas reservoirs in the Gulf of Mexico.
In general, seismic exploration prospects in a display of seismic data can be categorized as either structural or stratigraphic. The former are generally more obvious in displays of the seismic signals while the latter can be subtle and hard to find. Structural prospects relate to the general disposition, altitude, arrangement, or relative position of formation masses depicted by reflection events in the seismic data; whereas, stratigraphic prospects relate to predicting lithology, depositional environment, and interstitial fluids based on further evaluating and interpreting the reflection events themselves. The current trend in exploration geophysics is to acquire seismic data having higher frequency content in order to achieve better resolution of both subtle stratigraphic and structural prospects depicted in displays of the seismic signals.
After appropriate preprocessing of seismic data, generally two types of seismic sections or displays of the seismic signals can be produced. The first are automatic gain control (AGC) seismic sections for mapping structure, and the second are true amplitude seismic sections for evaluating stratigraphic prospects such as anomalous amplitude events, i.e., bright spots. The frequency content of true amplitude seismic sections is generally made as broad as possible in the belief that increasing the frequency content will provide better resolution of subtle stratigraphic prospects.
Changes in elastic impedance from one formation to another result in the imparted seismic energy being reflected and refracted. Generally, exploration geophysicists have assumed such changes in elastic impedance are step-like changes such as A and D in the display of impedance Z as a function of time shown in FIG. 1. As a consequence, the apparent amplitude of reflection events A' and D' in synthetic seismic signals T associated with such step-like changes in elastic impedance are generally equal to the reflection coefficient. Moreover, increasing the frequency content of the imparted seismic energy has been shown to improve the resolution of reflection events associated with such step-like changes in elastic impedance.
However, we have found that in the case of formations having a gradational change in elastic impedance, such as represented by B and D in the impedance function Z in FIG. 1, the use of higher-frequency content seismic energy can result in the apparent amplitudes of corresponding reflection events B' and C' in synthetic seismic signals T which are much less than one would have expected from a step-like change in elastic impedance having the same overall change as the gradational change in elastic impedance. Consequently, high resolution seismic acquisition techniques are not best suited for revealing stratigraphic prospects associated with gradational changes in elastic impedance. However, we have discovered novel methods for identifying and evaluating stratigraphic prospects in the seismic data associated with gradational changes in elastic impedance from those associated with step-like changes which advantageously employ the apparent liability of high resolution seismic acquisition techniques, as shall be more fully described below.